5 minute read
WHITE-FRINGED WEEVIL
At a recent meeting with growers the subject of white-fringed weevil came up, an invasive pest that we have had for a long time in New Zealand. It was a good reminder of a potentially serious threat, so I wanted to share some of that discussion here.
By Kate Trufitt : Potatoes New Zealand chief executive
The white-fringed weevil (Naupactus leucoloma) has been present in New Zealand since 1944, having originated in South America before spreading globally. While its impact in New Zealand has been relatively minor to date, the experience of our counterparts in Australia means we should stay alert to the threat – particularly with changing climatic conditions and reduced conventional chemistry options. Looking at growers in south-western Australia provides valuable insights into potential management strategies. Tasmania did not have the weevil until the mid-1980s, but it became a major problem by the 1990s.
The real damage comes from below ground. The larvae, which can take between eight and 24 months to develop fully, feed voraciously on lateral and taproots. This root damage doesn’t just reduce plant vigour; it can lead to plant death and leaves crops vulnerable to secondary root diseases. White-fringed weevil larvae feeding on the tubers can quickly affect up to 30 percent of the crop, which results in their rejection for market or the abandonment of large parts of the crop.
While adult weevils do feed on leaves, this causes minimal economic damage compared to the destruction wrought by their larvae. The pest is known to feed on at least 385 different plant species, with vineyards reporting damage during establishment as weevils ring-bark and feed on the roots of young vines. The Bragato Research Institute in Blenheim has included this pest in their monitoring studies among other weevils such as the garden weevil, recognising its potential threat to the wine industry.
Winter soil sampling can help determine if weevil densities are high enough to cause economic damage, though this is not straightforward. What makes white-fringed weevil larvae particularly troublesome is their unpredictability and the patchy distribution of infestations, which can be misinterpreted during soil sampling. The presence of similar-looking weevil species can also lead to both false positives and negatives in sampling efforts.
Spraying the crop with insecticides has proven largely ineffective against the larvae and there is evidence of resistance to conventional control methods.
Rather than risking their crop, Australian potato growers commonly apply pre-plant insecticide sprays as insurance against weevil damage. That is not common practice here in New Zealand. In any case, some of the more efficacious chemistry is unavailable. Chlorpyrifos is currently being reassessed by our Environmental Protection Authority and has already been banned in the European Union and the United States. Potato growers in New Zealand have already transitioned away from chlorpyrifos, seeking alternative management options.
Cultural controls provide some protection. Crucially, we should avoid planting susceptible crops in fields previously sown to legumes, which these pests favour. Rotary hoeing can expose larvae to birds. Long rotations with unsuitable hosts such as cereals and grasses can significantly reduce established weevil populations. One Australian grower found success using oats as a break crop, which not only reduced weevil numbers but improved soil health, addressing issues with Rhizoctonia and nematodes.
Fortunately, we are not facing this challenge alone. Potatoes New Zealand maintains close ties with our counterparts in Australia, giving us access to expertise developed through decades of weevil management practices. Visiting experts from Australia like Dr Paul Horne have shared valuable insights, while we also have in-house expertise with Potatoes New Zealand’s technical manager Dr Iain Kirkwood, who was closely involved in grower extension on sampling methods during his time at the Tasmanian Institute of Agriculture.
As our Australian neighbours have discovered, integrated approaches combining careful monitoring, break crops, and soil health improvement offer a sustainable path forward. Nevertheless, the experience in Australia also shows the urgent need for innovation and regulatory reform. We know that researchers and companies around the world are investing in crop protection innovation, biological controls, variety development, and other technologies to tackle threats like white-fringed weevil. Our growers here in New Zealand must have timely access to them.
By staying vigilant and learning from regions where this pest has already established as a major problem, we can protect New Zealand’s potato industry. If you have questions about white-fringed weevil or soil sampling, please contact us.
If you have any questions, please contact Potatoes New Zealand.
Phone: 0800 399 674
Email: info@potatoesnz.co.nz
Website: www.potatoesnz.co.nz
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